1. Field of the Invention
The present invention relates to a new method for brazing for example a connecting piece of electrically conducting material, for example of metal, to a metal surface by means of a new type of temperature-controlled brazing, whereby for certain types of material, for example steel, a martensite-free brazing is obtained, i.e. a brazing is produced without any deleterious structural changes (martensite formation). A brazing is obtained that underneath the brazed joint is completely free of martensite formation in for example railway track (rails) and/or piping. The present application also describes an arrangement for carrying out the method.
2. Description of the Related Art
Developments in railway traffic involve ever higher speeds and heavier axle loads. This in turn places increasing demands on the strength of railway track and its ability to withstand wear and accordingly rails are manufactured from higher-alloyed steel in order to meet these more stringent requirements. The material from which rails are manufactured is sensitive to thermal influences that can cause structural changes known as martensite formation (hardening effect).
Martensite formation may lead to crack formation in the rail material and due to the higher loads the rail may fracture, with catastrophic consequences for railway traffic. Consequently it is very important to braze signal and other wiring and cabling firmly to the rail by using a method that does not cause martensite formation in the rail.
Up to now it has only been possible to minimise martensite formation or structural changes by means of a pin brazing method, which is described in Swedish patent 9003708-6 (469 319). Hitherto it has not been possible by any pin brazing method to completely eliminate martensite formation in electrical contact connections that are intended to join two or more objects by means of an electrical connection.
The most serious problem in current methods employed on railway track is the large amount of heat that is produced underneath the brazing joint, which is caused by the electric arc that is generated in the brazing process and produces a deleterious structural change or martensite formation.
Another problem has been the electrode per se used in the process, which is discharged from a brazing pin. The brazing pin contains both flux material and brazing metal and at the start of the process an electric arc is ignited and flux flows from the brazing pin to the brazing joint and cleans the latter. Following this brazing material flows from the brazing pin to the brazing joint. Finally, the brazing pin is pressed downwardly into the brazing melt and penetrates the brazing joint. In order to produce a connection between the metal surface on the rail/piping and cable shoe, at the present time a cable shoe is provided with an opening so that flux material and brazing material can pass from the brazing pin through the cable shoe and then firmly braze the cable shoe to the flat or curved metal surface. At the start of the brazing process the electric arc acts directly on the rail and produces a very high local temperature which is then transmitted indirectly via the brazing melt and generates high temperatures directly in the rail, which have a deleterious effect on the latter. There is also the risk of an alloying of electrode material in the brazing when using conventional pin brazing systems, which has adverse effects on both the brazing and workpiece. The final stage in the currently employed pin brazing involves pressing the brazing pin downwardly into the brazed joint and then breaking off the pin, which has a negative effect on the brazed joint itself.
Another practical problem with conventional brazing processes is the poor grounding of workpieces, which causes a large amount of unsuccessful brazing. With conventional pin brazing the rail constitutes the opposite pole or terminal to the electrode. This requires special operating procedures and separate equipment. For this reason it is desired to be able to make changes as regards the brazing process.
Furthermore, a disadvantage of current brazing processes is the cable shoe, which is fastened by means of brazing pins containing flux material as well as brazing material. The cable shoes that are currently employed have a flat end with a hole which enables brazing to flow therethrough from the upper side of the cable shoe and then secure the latter to a metal surface. Since the brazing process takes place rapidly a large amount of heat is required, which is transmitted from the electric arc via the brazing through the cable shoe to the workpiece/rail. The problem has been that a satisfactorily secure brazing has to be produced at the same time without causing any structural change underneath the brazed joint. Up to now it has only been possible to minimise martensite formation by the process described in Swedish patent 9003708-6.